Most internal combustion engines include a timing transmission in which an endless chain delivers power from a driving sprocket to a driven sprocket. The sprockets have been typically produced by sinter molding of a powder alloy or by skiving bulk steel. However, more recently, because of diverse user requirements, such as improvement of quality and production, cost reduction, short delivery time, and the like, sprockets have been produced by fine blanking of steel plate. Such a process is described in Japanese Laid-Open Patent Publication No. 2002-1449.
Fine blanking is a precision process in which a material to be blanked or punched is constrained so that the material is sheared while compression force is applied to the material from all directions. In fine blanking, a workpiece is held under a high pressure against a die by means of a V-ring, and a portion is punched out of the workpiece by means of a punch, while a counter force is applied by a reverse punch back-up. The fine blanking process is capable of producing parts which are flatter, more uniform, and dimensionally more accurate, and which have a cleaner shear face, than parts produced by conventional stamping.
In the fine blanking process, as illustrated by FIGS. 7(a), 7(b) and 7(c,) a phenomenon known as “penetration,” produces a curved portion 260 and a protruding portion 250, as seen in FIG. 7(c), which is a detailed view of a peripheral part of the sprocket shown in FIGS. 7(a) and 7(b). Thus, after fine blanking, another fabrication step, such as turning, milling, or the like, is required to shave or remove the protruding portion. Then, a further finishing step is required, at the location where the protruding portion was removed, in order to provide the front and back sides of the tooth head with the same round cross-section corresponding to the shape of the curved portion 260. Even when the tooth head is finished so that has a symmetric curvature, the process of turning, milling, or the like, can leave a burr at a tooth bottom or on a tooth side, which can cause damage or excessive wear to a chain that engages the sprocket.